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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 25 Jun 2019

Submitted as: research article | 25 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, monitored by multiparametric data

Ayleen Gaete1, Thomas R. Walter1, Stefan Bredemeyer1,2, Martin Zimmer1, Christian Kujawa1, Luis Franco3, Juan San Martin4, and Claudia Bucarey Parra3 Ayleen Gaete et al.
  • 1GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany
  • 3Observatorio Volcanológico de Los Andes del Sur (OVDAS), Servicio Nacional de Geología y Minería (SERNAGEOMIN), Temuco, Chile
  • 4Physics Science Department, Universidad de la Frontera, Casilla 54-D, Temuco, Chile

Abstract. Small steam-driven volcanic explosions are common at volcanoes worldwide but are rarely documented or monitored; therefore, these events still put residents and tourists at risk every year. Steam-driven explosions also occur frequently (once every 2–5 years on average) at Lascar volcano, Chile, where they are often spontaneous and lack any identifiable precursor activity. Here, for the first time at Lascar, we describe the processes culminating in such a sudden volcanic explosion that occurred on October 30, 2015, which was thoroughly monitored by cameras, a seismic network, and gas (SO2 and CO2) and temperature sensors.

Prior to the eruption, we retrospectively identified unrest manifesting as a gradual increase in the number of long-period (LP) seismic events in 2014, indicating an augmented level of activity at the volcano. Additionally, SO2 flux and thermal anomalies were detected before the eruption. Then, our weather station reported a precipitation event, followed by changes in the brightness of the permanent volcanic plume and (10 days later) by the sudden volcanic explosion. The multidisciplinary data exhibited short-term variations associated with the explosion, including (1) an abrupt eruption onset that was seismically identified in the 1–10 Hz frequency band, (2) the detection of a 1.7 km high white-grey eruption column in camera images, and (3) a pronounced spike in sulfur dioxide (SO2) emission rates reaching 55 kg sec−1 during the main pulse of the eruption as measured by a mini-DOAS scanner. Continuous CO2 gas and temperature measurements conducted at a fumarole on the southern rim of the Lascar crater revealed a pronounced change in the trend of the relationship between the carbon dioxide (CO2) mixing ratio and the gas outlet temperature; we believe that this change was associated with the prior precipitation event. An increased thermal anomaly inside the active crater observed through Sentinel-2 images and drone overflights performed after the steam-driven explosion revealed the presence of a fracture ~ 50 metres in diameter truncating the dome and located deep inside the active crater, which coincides well with the location of the thermal anomaly. Altogether, these observations lead us to infer that a lava dome was present and subjected to cooling and inhibited degassing. We conjecture that a precipitation event led to the short-term build-up of pressure inside the shallow dome that eventually triggered a vent-clearing phreatic explosion. This study shows the chronology of events culminating in a steam-driven explosion but also demonstrates that phreatic explosions are difficult to forecast, even if the volcano is thoroughly monitored; these findings also emphasize why ascending to the summits of Lascar and similar volcanoes is hazardous, particularly after considerable rainfall.

Ayleen Gaete et al.
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Ayleen Gaete et al.
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Publications Copernicus
Short summary
Phreatic eruptions often occur without signs of enhanced volcanic unrest, eluding from detection and posing in threat to people in the vicinity. We analyzed data of the 2015 phreatic eruption of Lascar volcano, Chile to retrospectively identify a rainfall as the trigger mechanism, and potential signs heralding this minor eruption. We showed that it is possible to detect precursory activity of phreatic eruptions even if deep-seated activity, by deploying appropriate multiparametric monitoring.
Phreatic eruptions often occur without signs of enhanced volcanic unrest, eluding from detection...